Field of the Invention
The present invention relates to a process for the continuous determination of the interaction between drilling fluids and shale formations.
More specifically, the present invention relates to a process for continuously verifying the effect of drilling fluids on the stability of shale formations in oil well drilling, by measuring the ultrasonic wave transmission velocity on clay cuttings subjected to aging in drilling fluids.
During the drilling of an oil well, there is often the problem of instability of the well in shale formations. To prevent this instability, drilling fluids are prepared with additives suitable for maintaining (or if possible improving) the mechanical properties of the shales and inhibiting swelling. The effectiveness of these additives is evaluated by tests which are specified in API procedures, for example, dispersion tests of clay cuttings in mud (hot-rolling test) or laboratory procedures (for example measuring the swelling of clay samples). On the other hand, techniques for evaluating the mechanical characteristics (for example uniaxial or triaxial creep tests) have the great disadvantage of being destructive, costly and time-consuming.
None of these techniques considers the possibility of analyzing the mechanical behavior of shale interacting with a drilling fluid in relation to time, a fundamental factor in evaluating the arising of instability in shale formations during the drilling of oil wells.
It is also known that the relative mechanical properties can be determined from the velocity of sound waves through rock samples. Although these acoustic techniques are not destructive, they have the drawback of being used at present only for the acoustic characterization of formations and although they detect the mechanical properties of the rock, they neglect, on the other hand, the effect due to rock-drilling fluid interaction.
A process has now been found which overcomes the above disadvantages, as it allows the mechanical properties of shales to be evaluated in the presence of drilling fluids in relation to the interaction time. The process of the present invention also has the advantage of not being destructive.
In accordance with this, the present invention relates to a process for the continuous determination of the inter-action between drilling fluids and shale formations, which comprises:
(a) preparation of a water- or oil-based drilling fluid;
(b) preparation of a shale sample having at least two flat, parallel opposite surfaces;
(c) preparation of the mixture of (b) and (a);
(d) continuous measurement of the ultrasonic wave transmission velocity through the sample (c), and variations in thickness (swelling and shrinking) of the shale sample, due to interaction with the drilling fluid (a).
The process of the present invention can be applied to cuttings produced during drilling, or to shale samples prepared by suitably cutting well or outcrop cores. Alternatively, it is possible to operate on reconstituted shale samples.